In the field of wireless mobile communication, there are proposed techniques of compensating a nonlinear distortion of a power amplifier used in a base station or the like. In a case of amplifying signals in a plurality of frequency bands concurrently by a common power amplifier, one of the techniques compensates a nonlinear distortion by using a plurality of distortion compensation units and a dual-band DPD (Digital Pre-Distortion). Further, there is a power amplifier using the dual-band DPD, which includes a dual-input truncated Volterra model digital filer in the preceding stage of a two-dimensional LUT (Look Up Table) and also compensates a second-order distortion. With regard to harmonics or intermodulation distortions generated in another band, there are a technique of performing distortion compensation by adding a signal with inverse characteristics and a technique of performing distortion compensation by solving nonlinear simultaneous equations.
However, the techniques described above are difficult to obtain sufficient distortion compensation performance, and may cause deterioration of radio quality. For example, in a case of amplifying signals in a plurality of frequency bands concurrently by a common power amplifier, even-order distortions may be generated at a frequency corresponding to a difference between center frequencies of two bands or at a frequency that is twice a lower one of the center frequencies under a condition where one of the center frequencies of the two bands is about twice the other center frequency (for example, 4.6 GHz and 9.0 GHz). Further, even in a case where signals in a plurality of frequency bands are amplified concurrently by a plurality of power amplifiers and are combined with each other, if isolation of a combiner is insufficient, even-order distortions may be generated similarly. When these even-order distortions are generated at a frequency close to a band of a transmission signal, it is difficult to cut the even-order distortions by a filter. Even if cutting by a filter is possible, a steep filter is needed, which increases the circuit scale of a device. In particular, in a case where even-order distortions are generated in a band of a transmission signal, cutting by a filter is very difficult.
Therefore, a distortion compensation apparatus has been proposed, which suppresses even-order distortions in addition to odd-order distortions generated in a case of amplifying a multi-band signal by a power amplifier. This apparatus includes both a distortion compensation unit that compensates odd-order distortions and a distortion compensation unit that compensates even-order distortions. This apparatus employs a configuration in which, when a dual-band signal in a band A (a center frequency fA) and a band B (a center frequency fB) is subjected to common amplification (fA<fB), even-order distortions generated at a center frequency of 2fA and a center frequency of fB−fA are compensated in addition to odd-order distortions generated in the bands A and B.
Next, problems of the related distortion compensation apparatus described above are explained with reference to FIG. 7. FIG. 7 is an explanatory diagram of the problems of the related distortion compensation apparatus. The distortion compensation apparatus uses a local oscillator source with a frequency of fLO1 that is common to a transmission side and a feedback side for a band A (a center frequency fA) and a local oscillator source with a frequency of fLO2 that is common to a transmission side and a feedback side for a band B (a center frequency fB). In general, a frequency shift (Δf1, Δf2) is momentarily generated in the local oscillator source because of a phase fluctuation. However, the frequency shift on the transmission side and that on the feedback side are canceled out. Therefore, the frequency shift exerts no influence. In a low-band side path on the transmission side, which is illustrated as an upper path, the distortion compensation apparatus outputs a band-A transmission signal with a frequency of fA−fLO1 from a DAC (Digital to Analog Converter), and converts the frequency of that transmission signal by an up converter with a local frequency of fLO1+Δf1 to a frequency of fA+Δf1. Similarly, in a high-band side path on the transmission side, which is illustrated as a lower path, the distortion compensation apparatus outputs a band-B transmission signal with a frequency of fB−fLO2 from a DAC, and converts the frequency of that transmission signal to a frequency of fB+Δf2 by an up converter with a local frequency of fLO2+Δf2. These radio frequency (RF) signals are combined with each other by a signal combiner. The resultant signal is subjected to power amplification by an amplifier.
On the feedback side, a feedback signal that is a portion of an amplifier output and is obtained by a coupler, is distributed into two signals by a distributor. In an upper path, that is, a low-band side path, the distortion compensation apparatus allows a feedback signal in the band A (a center frequency fA+Δf1) to pass through an LPF (Low Pass Filter) and converts the frequency of that feedback signal to a frequency of fA−fLO1 by a down converter with a local frequency of fLO1+Δf1. The converted signal is converted from an analog signal to a digital signal by an ADC (Analog to Digital Converter). In a lower path, that is, a high-band side path, the distortion compensation apparatus allows a feedback signal in the band B (a center frequency fB+Δf2) to pass through an HPF (High Pass Filter) and converts the frequency of that feedback signal to a frequency of fB−fLO2 by a down converter with a local frequency of fLO2+Δf2. The converted signal is converted from an analog signal to a digital signal by an ADC. In this manner, with respect to a main signal, the distortion compensation apparatus uses the local oscillator source with the frequency of fLO1 that is common to the transmission side and the feedback side for the band A and the local oscillator source with the frequency of fLO2 that is common to the transmission side and the feedback side for the band B. Therefore, the frequency of a DAC output and the frequency of an ADC input match each other perfectly, so that influence of a frequency shift caused by a phase fluctuation of the local oscillator source is eliminated.
However, even-order distortions generated at a center frequency of 2fA and a center frequency of fB−fA are affected by the above frequency shift. Frequencies of even-order distortion compensation signals are 2fA+Δf2 and fB−fA+Δf1, respectively. Meanwhile, in a case where the frequency of the band A is fA+Δf1 and the frequency of the band B is fB+Δf2, frequencies at which the even-order distortions are actually generated are 2fA+2 Δf1 and fB−fA+Δf2−Δf1, respectively. Therefore, a condition where the frequencies at which these even-order distortions are generated and the frequencies of the above even-order distortion compensation signals match each other is 2 Δf1=Δf2. However, in general, two independent local oscillator sources do not always satisfy the condition of 2 Δf1=Δf2. Therefore, shifting is generated between the frequency at which even-order distortions are actually generated and the frequency of the even-order distortion compensation signals, so that a phenomenon appears in which even-order residual distortions move up and down over time in a spectrum waveform of an output of a power amplifier after distortion compensation. Therefore, an effect of suppressing the even-order distortions is lowered because of the influence of the local phase fluctuation. Consequently, distortion compensation performance in common amplification of multiple bands is lowered.